Oxide catalysts defect sites

The stronger Lewis-type acidity in the equilibrated catalyst agrees with the many literature evidences about the existence of these sites in well-crystallized VPP (14). Also, a discrete amount of ions may contribute to an enhancement of Lewis-type acidity, due to the presence of defects associated to the anionic vacancies (5). The Bronsted acidity in the oxidized catalyst (sample ox3sp) can be associated to... 

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

Over most reported Au catalysts, CO oxidation takes place at the junction perimeter between Au NPs and the metal oxide supports. Carbon monoxide is adsorbed on the edges, corners and steps of Au NPs. Molecular oxygen is adsorbed on the support surfaces and may be activated at the oxygen defect sites at the perimeter interfaces, where the two adsorbates react to form C02 in the gas phase. At the perimeter interfaces Au is assumed to exist as Au3 +, which might be stabilized through bonding with OH-. 

The incorporation of Ti into the MOR structure was confirmed by the appearance of the specific absorption band in the IR spectra [20]. Very recently, Kubota et al. have synthesized Ti-YNU-2 (MSE) [25] by the post-synthesis modification ofYNU-2 (P) that has a large number of defect sites [87]. Ti-YNU-2 has proved to be a very active catalyst in liquid-phase oxidation using H202 as oxidant [25]. 

More than 90% of the natural metallic elements of the periodic table form perovskites the wide range of cations, the possibility of partial substitution of A or B cation sites, and the remarkable capacity to accommodate a multitude of different kinds of defects result in a wealth of properties of these solids leading to applications ranging from superconductors (33) to oxidation catalysts (34). 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

---